Electrical connection boxes that are to be mounted on an automobile house, for example, a wiring board on which electronic components are mounted, and an insulation board to which a bus bar is attached. As disclosed in JP 2008-172893A, for example, there is an electrical connection box known as one type of such electrical connection box, in which an electronic control unit that houses a control circuit, such as an ECU, is detachably attached to an electrical connection box main body as a separate attachment. Such an electrical connection box has an advantage in that the electronic control unit is a separate structure and therefore it is easy to replace or change the electronic control unit, for example.
Such an electrical connection box that includes an electrical connection box main body and an attachment structure has connectors, which are respectively provided at corresponding positions on an attaching surface of the electrical connection box main body and an alignment surface of the attachment that is to be aligned with the attaching surface. Specifically, an opening is provided in the attaching surface of the electrical connection box main body so as to penetrate therethrough, and a portion of a main body-side connector protrudes to the outside via the opening. The main body-side connector is provided on an internal circuit board so as to protrude therefrom. An attachment-side connector is provided on the alignment surface of the attachment so as to protrude therefrom. When the attachment is to be attached to the electrical connection box main body, the main body-side connector and the attachment-side connector are connected to each other, and thus electrical circuits provided on the electrical connection box main body and electrical circuits provided on the attachment are interconnected so as to be conductive.
However, in such an electrical connection box with a conventional structure, an opening is provided in the attaching surface of the electrical connection box main body, and therefore, when the electrical connection box gets wet, there is the risk of water entering the inside of the connector via the opening. Since the opening of the electrical connection box main body is to be covered by the attached attachment from the outside, no specific drainage structure has been provided to drain water that has entered from the opening. Therefore, there is the risk of a failure such as a short circuit occurring due to the internal circuit board getting wet when water, which has reached the opening of the electrical connection box main body via a gap between opposed surfaces of the electrical connection box main body and the attachment, enters the inside from the opening.